1. Field of the Invention
The present invention relates to a small and thin push button switch, particularly to its stem.
2. Description of the Related Art
A stem assembled in a push button switch has a combined configuration of a main body and its surrounding flange. The stem is mounted on a housing so that the stem is movable through an opening of the housing in an up-and-down direction. Such a stem serves to push down a movable contact to contact with a fixed contact disposed so as to face a movable contact. The fixed contact is exposed on a bottom surface of the housing. Generally, a stem 1 is made of synthetic resin, and consists of a flange portion 1a and a main body 1b integrated together, as shown in FIG. 8.
As another example of the stem, as shown in FIG. 9, there is known a stem 2 consisted of a metallic flange made by a metallic plate 3 having a through hole 3a, and a synthetic resin main body 4.
However, in order to promote reduction of the thickness of the small push button switch, reducing the thickness of flange accomplishes this purpose. In the case that the stem 1 is made of only synthetic resin as shown in FIG. 8, it is difficult to reduce the thickness of the flange 1a, since melted synthetic resin cannot be smoothly filled in a thin cavity during molding.
On the other hand, though the stem 2 shown in FIG. 9 has a thin metallic flange, manufacturing steps for obtaining the stem 2 are complicated. Namely, it is necessary, first of all, to obtain a metallic plate 3 pressed into a flange configuration. Next, this metallic plate 3 is inserted in dies to obtain the main body 4 of synthetic resin material associated with the metallic plate 3. That is, it requires two manufacturing steps, and production efficiency is low.
Furthermore, this stem 2 has a poor thermal durability, since the metallic plate 3 and the synthetic resin main body 4 have different thermal expansion coefficients. Thus, it is feared that the metallic plate 3 comes out of from the synthetic resin main body 4. In addition, there is a problem such that traces of injection gates; i.e. projections, are formed on the surface of the synthetic resin main body 4. As a result, reliability of stem 2 is low.